In-situ capping of underwater contaminants involves the placing of a subaqueous cover over contaminated sediments to stabilize sediments, minimize their re-suspension and transport, and reduce dissolved contaminant transport into surface waters.
In past applications, in-situ capping has typically been constructed with either, a) loosely placed clean sediment, silt and sand; b) bulk reactive material such as organoclay, or c) a low permeability liner such as geomembrane or geosynthetic clay liner; or d) reactive geocomposite mat.
Disadvantages of loosely placed clean sediment, silt or sand are: low carbon content requires relatively thick cap; the material may segregate upon settling resulting in stratified layers; benthic organisms may burrow into the material.
Disadvantages of bulk reactive material are: the angle of repose requires material on the sides beyond the area of concern, and difficulty in placing the material through water.
A disadvantage of low permeability liners is that they may be subject to uplift from gases generated by the sediment below the liner.
The prior art is replete with methods and articles used to confine or store a wide variety of environmental contaminants ranging from completely capping, in-situ, contaminated sediments that are left in-place in underwater environments; terrestrial landfills wherein dredged or otherwise collected contaminated sediments are placed within an engineered disposal site surrounded with an impervious liner system and capped with an impervious material; and the use of a reactive mat and/or reactive backfill that surrounds the contaminated material. Examples of reactive geocomposites and mats are found in U.S. Pat. No. 6,284,681 B1 ('681) and published application US 2002/0151241 A1 ('241). The reactive geocomposites described in these two publications include one or more layers of reactive material each surrounded by outer geotextiles that allow contaminated liquid to pass through the reactive layer for sorption or reaction of the contaminants with a reactive material contained between the outer geotextile layers, and in the case of the '241 published application, the geocomposite may be deployed vertically.
One of the major problems encountered with the use of reactive mats for controlling or confining contaminated materials, or in controlling or preventing leaching of contaminants from sediments and preventing the contaminants from entering ground water supplies, and particularly from ground water traversing upwardly through a lake or ocean soil interface into the lake or ocean, is in the ability to provide a transportable, integral mat having a sufficient volume or thickness of reactive material so that the mat provides very long term protection without the necessity of periodic replacement. The reactive mats described in the '681 patent and in the '241 publication provide alternating layers of geotextile/reactive material/geotextile/reactive material since a sufficient thickness of reactive material cannot be provided in a single reactive material core layer without that reactive material being lost during transportation or installation. The mat disclosed in U.S. Pat. No. 7,128,498 does not have adjacent layers interconnected but relies upon a rock covering layer to maintain the mat in position. The mat disclosed in the '498 patent has separate layers that are not structurally interconnected and may easily separate due to the powerful hydraulic force experienced with lake and river ground water supplies. Also, deployment of separate geotextiles and reactive materials through a water column is difficult.
This assignee's U.S. Pat. Nos. 5,237,945 ('945) and 5,389,166 ('166) describe the manufacture of a water barrier formed from a clay-fiber mat that may include a powdered or granular bentonite clay, a powdered or granular liquid-interacting material, e.g., a contaminant-reactant, or providing the contaminant-reactant as a separate layer in the water barrier product. The water barrier mat formed in accordance with the '945 and '166 patents is manufactured by laying down geosynthetic fibers and the water swellable clay, with or without the contaminant-reactant material, simultaneously. In this manner, a geosynthetic composite material can be manufactured wherein the geosynthetic fibers are surrounded by the water-swellable clay, with or without the contaminant-reactant material, in initially forming a relatively thick geotextile that essentially prevents water flow-through (maximum water-permeability of 1×10−7 cm/sec.). Such a mat must be subsequently consolidated and the fibers must be substantially densified after the initial formation of the mixture of powdered or granular material and fibers in an attempt to secure the fibers in position surrounding the powdered or granular material.